As is well known, it is important in the fabrication of semi-conductor devices to know the thickness of an epi layer on a semi-conductor wafer. Different methods are known within the prior art for measuring or determining the epi thickness including methods based upon IR interference physical optic theory. In accordance with such theory, IR energy is directed onto a wafer and is reflected from the surface of the epi layer and from the interface between the epi layer and underlying substrate. The IR energy is directed as an incident beam onto a small area of the wafer at a position where the epi thickness is to be measured. Such incident beam is divided to form two reflected beams. One beam is reflected from the surface of the epi layer and the other beam is reflected from the epi layer/substrate interface. The two reflected beams interface with each other in such a manner that the epi thickness can be determined by spectral reflectance and interferometric methods.
The spectral reflectance method is based on the phenomena that the degree of optical interference between the two reflected beams cyclically varies at each wavelength across a spectrum. The variation produces a series of maxima and minima reflectance values in accordance with the degree of constructive and destructive interference at the different wavelengths. Such method generally involves measuring the spectral reflectance and then calculating the thickness using the reflectance at two different maxima or minima. An example of this method is standard test method F95 of the American Society for Testing Materials (ASTM) for "Thickness of Epitaxial Layers of Silicon on Substrates of the Same Type by Infrared Reflectance".
In the interferometric method, an interferometer is used to generate an interferogram from the two reflected beams. The interferogram includes a center burst or peak and two side bursts or peaks created as a result of displacement of the interferometer mirror. In a perfect system, the interferometer would be symetrical and the degree of mirror displacement between two positions corresponding to two of the bursts or peaks, is proportional to the epi thickness. In actual practice however, the interferogram is asymmetrical and a double Fourier Transform and other mathematical manipulations are performed to create an idealized interferogram, from which the thickness is calculated as a function of mirror displacement between the side peaks. An example of this method is described in "Measurement of Si Epitaxial Thickness Using a Michaelson Interferometer", Paul F. Cox and Arnold F. Stalder, J. Elec. Soc.: Solid State Science and Technology, February 1973, pgs. 287-292.
A current trend in the semi-conductor industry is to grow thinner and thinner epi layers having thicknesses less than one-half a micron. Thus there has developed the need to measure the thicknesses of such thin layers. But the methods and apparatuses of the prior art, particularily commercially available instruments, appear to be limited to making accurate measurements only for thicknesses substantially above 0.5 microns. In the spectral reflectance method, the number of extrema decrease with a decrease in epi thickness and it becomes difficult or impossible to average a number of calculations per the ASTM method, or even to recognize the extrema. In the interferometric method, the side peaks interfere with the center peak below three microns and with each other at thinner thicknesses, so that the mirror displacement cannot be accurately determined.
Additionally, prior art theory appears to be based on a number of simplifying assumptions about some of the optical factors or conditions, which assumptions are not exactly true and which tend to produce inaccurate results when applied to the measurement of extremely thin epi layers. Examples of such assumptions are that the index of refraction is constant, that there is no phase shift at the epi layer/substrate interface, and that the substrate is non-absorbant.